1. Technical Field
The present inventions relate to switching amplifiers and, more particularly, relate to switching amplifiers employing pulse width modulation and electromagnetic interference reduction.
2. Description of the Related Art
Switching amplifiers have been used to power an audio speaker or transducer. One application for a switching amplifier has been the audio amplifier in an automobile sound system.
Switching amplifiers can cause electromagnetic interference (EMI). Several approaches have been employed to reduce EMI emitted from switching amplifiers. Among others, these have included, first, analog pulse width modulation (PWM) with a dithered switching frequency and, second, spread spectrum digital PWM.
Both analog and digital pulse width modulation (PWM) with a dithered switching frequency has been used to reduce EMI in switching amplifiers. Digital PWM with dithered switching frequency has better performance and greater suitability for high performance switching audio amplifiers at a mass market cost.
Spread spectrum digital pulse width modulation (PWM) has been used to reduce EMI in switching amplifiers. Spread spectrum digital PWM relies on the cancellation of the PWM carrier frequency in the digital integral noise shaping (INS) loop of a full bridge power stage of a switching amplifier.
U.S. Pat. No. 7,130,346 entitled “Method and apparatus having a digital PWM signal generator with integral noise shaping” and issued Oct. 31, 2006 to Midya et al. has a common inventor with the present inventions. This patent describes a spread spectrum digital PWM method that works in a full bridge power stage switching amplifier. A pair of PWM signals is fed back from respective sides of a full bridge power stage. An integral noise shaper needed this pair of PWM signals to operate.
U.S. Patent Publication No. 20080252392 entitled “Discrete Dithered Frequency Pulse Width Modulation” was published on Oct. 16, 2008 by Midya et al. and has a common inventor with the present inventions. It is copending with the present application. It discloses a switching amplifier using spread spectrum digital PWM in a full bridge power stage.